Michael Keating’s 11-year prison sentence for methamphetamine production was supposed to end next June. But he is one of thousands of federal prisoners who are benefiting from a reduction in drug sentences, and on Friday he became an ex-prisoner more than seven months earlier than he ever had reason to expect.

Mr. Keating, 32, celebrated his first day of official freedom by sharing steak and potatoes with his mother and siblings at the family farmstead in Winfield, Mo.

“It feels like a lot of weight has been lifted off my shoulders,” said Mr. Keating, who spent close to a decade behind bars and the last several months, after his sentence was shortened, in a halfway house and then under house arrest.

From Friday to Monday, more than 6,000 federal prisoners will be released earlier than they once expected, although nearly one-third of them are foreign citizens who will be handed over for likely deportation.

Over the next few years, as a result of an across-the-board adjustment of federal drug penalties by the United States Sentencing Commission, tens of thousands more inmates may benefit from reductions in their terms, and new sentences will be somewhat shorter than they were in recent decades.

Mr. Keating paged through a photo album he made while serving time at various prisons in the Midwest. Credit Kile Brewer for The New York Times

The shift reflects concerns about the severe overcrowding and expense of federal prisons and, even more, the widely shared sense among many leaders of both political parties as well as criminologists that the harsh federal penalties of the war on drugs were often too extreme. President Obama has declared “criminal justice reform” a priority and on Monday plans to visit Newark to “highlight the re-entry process of formerly incarcerated individuals,” the White House said Friday.

The initial group now being freed had sentences that originally averaged 10.5 years and were reduced to an average of 8.5 years, according to the Justice Department. The laws imposing mandatory minimum sentences of decades for some repeat or large-scale offenders remain in effect.

“People are still getting slammed pretty hard for drug offenses,” said Douglas A. Berman, an expert on criminal law at the Moritz College of Law of Ohio State University.

The releases result from decisions last year by the sentencing commission, an independent body that sets guideline ranges for federal sentences. Until 2005, those guidelines were mandatory; now they are advisory, but remain a starting point for judges.

Citing overcrowding in federal prisons, where half of all inmates are in for drug convictions, the commission in 2014 made downward adjustments in the scoring system for determining sentences and made the changes retroactive.

The commission said that all reductions in existing sentences must be approved by a judge, who can weigh the risks to the public. It said they also should be delayed by a year, until Nov. 1, 2015, to give the courts time to assess prisoners and the Bureau of Prisons time to prepare for their smooth release.

The result was a backlog of prisoners who otherwise could have been freed over the last 12 months. The judge cut Mr. Keating’s sentence by 15 months, for example, which would have allowed his release last May, but he had to wait until now.

While the change in sentencing guidelines falls far short of proposals to abolish harsh mandatory minimum sentences and reduce the felony prosecution of lower-level drug offenders, it has been welcomed by advocates of justice reform.

“I think it’s fantastic,” said Julie Stewart, the president of Families Against Mandatory Minimums. “People have been overpunished for decades, and this, this is slightly correcting the excessive sentences.”

The commission and the Justice Department say there is no evidence that keeping inmates in prison for longer terms affects their chances of returning to crime. When sentences for crack cocaine offenses were retroactively reduced in 2007, the commission found in a study that those released early had a slightly lower rate of recidivism than other federal inmates, though rates for both groups were more than 40 percent over five years.

But others say releasing offenders early will only increase crime. Bill Otis, a former federal prosecutor and outspoken opponent of softened drug laws, attacked the early-release program in a blog post on Friday. “When these people start up with a criminal life again, as we know in advance many and very likely most of them will,” he wrote, “who will be accountable for the release decisions, and who will pay the price for the harm then caused?”

Of 6,112 being released in the initial group, 1,764 are noncitizens who will transfer to the custody of immigration officials. Of the 4,348 others, nearly 80 percent are already outside prison walls, the Justice Department said — living in halfway houses which they may leave for jobs and counseling, or under home confinement that allows them to move about within strict limits.

Virtually all of those released will face years of supervision by a probation officer.

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The current bubble in releases has stretched the country’s halfway houses, with some prisoners not landing spots, and may temporarily strain probation offices, federal defenders and justice officials say. But they also note that more than 1,000 inmates are routinely released from federal prison each week, and more than 10,000 a week from state prisons.

A total of 46,000 current federal drug prisoners are eligible for resentencing, according to the commission, but so far, judges have turned down about one in four applicants.

In any case, researchers say, the effect on total federal prisoners, now some 205,000, will be small, pulling the total down by a few percentage points. Modest additional reductions in the federal prison population will be achieved if bills now before Congress, to reduce the use of mandatory minimums and other measures, are adopted.

Still, any cut in prison terms at all is precious to inmates, Mr. Keating and other former prisoners said. Mr. Keating knows he is luckier than many of his fellow prisoners, especially because he has a supportive family.

Mr. Keating was 22 and addicted to methamphetamine when he was arrested and charged with production because, he said before pleading guilty, he had allowed others to produce the drug on his land in return for a free supply.

He was shocked by his sentence of 11 years and three months, he said, but remained a model prisoner.

His greatest revelation came, he said, when he was able to move in March to a halfway house, which allowed him to start a job and visit his family on weekends. Since May, on house arrest, he has had to be home from work by 5:30 p.m., among other restrictions. But he has been able to keep what he calls his “dream job,” with a company that wires houses with utilities and entertainment controllable by cellphone.

He moved back into the house next to his mother’s and enjoyed his siblings’ babies, and he describes those recent months, awaiting an end to his status as a prisoner, as “the best time of my life.”

If the seven-month reduction in his term seems small compared with the more than nine years he spent behind bars, Mr. Keating said, think again.